1. Field of the Invention
The present invention relates to the field of wellbore production logging tools. More specifically, the present invention relates to a tool for measuring the density of a fluid filling a wellbore by means of measuring a differential pressure between two spaced-apart locations along the tool.
2. Discussion of the Related Art
Production logging tools are used in wellbores penetrating earth formations for, among other things, determining relative volumes of each one of a plurality of various fluids which may be entering the wellbore. An instrument which typically is included with production logging tools is a fluid density instrument. The fluid density instrument is used to make measurements related to the density, or specific gravity, of the fluids within the wellbore at a plurality of depths within the wellbore. Measurements of the density of the fluid filling the wellbore are typically combined with other measurements provided by the production logging tools to determine the relative volumes of each one of the plurality of fluids which may be present in the wellbore.
An instrument for measuring the density of the fluid in the wellbore is known in the art and is called a differential pressure fluid density (DPFD) instrument. A typical DPFD instrument is disclosed, for example, in U.S. Pat. No. 3,616,688 issued to Bonnet et al. The instrument in the Bonnet patent comprises a differential pressure transducer which is adapted to measure a difference in pressure between two spaced-apart locations along the tool. The difference in pressure measured by the transducer can be related to the density of the fluid disposed between the spaced-apart locations by the expression: ##EQU1## where .rho. is the density of the fluid, .DELTA.P is the measured differential pressure, g is the local acceleration due to gravity, and .DELTA.V is the vertical separation of the spaced-apart locations between which the differential pressure is measured.
The distance between the spaced-apart locations along the tool known in the art typically represents a compromise between the sensitivity of the instrument, which generally increases as the distance increases, and the maximum differential pressure to which the transducer can be exposed without destructive failure, because for any given density of fluid disposed between the spaced-apart locations the differential pressure across the transducer typically increases as the distance between the spaced-apart locations increases.
A limitation of the DPFD instrument known in the art is that the sensitivity of the instrument is reduced when used in certain wellbores which may be inclined from vertical, because the magnitude of the differential pressure developed between the spaced-apart locations is related to the vertical separation between the spaced-apart locations, as previously explained herein. In highly inclined wellbores the vertical separation between the spaced-apart locations typically is very small. The actual vertical separation between the spaced-apart locations at any value of inclination from the vertical can be expressed as: EQU .DELTA.V=H*cos(I) (2)
where .DELTA.V is the vertical separation between the spaced apart locations, H is the distance between the spaced-apart locations, and I is the angle of inclination of the wellbore from vertical (in a vertical wellbore I=0). It can be inferred from equation (2) that in wellbores approaching horizontal, the sensitivity of the DPFD tool known in the art is severely limited.
A further limitation of the DFPD instrument known in the art is that the sensitivity of the instrument can be reduced in fluids having densities nearly equal to the density of water, particularly fluids such as mixtures of crude oil and water having a large fractional composition of water, because small density differences generate differences in differential pressure across the transducer which may be below the resolution of the transducer.
It is an object of the present invention to provide a differential pressure fluid density instrument having a distance between spaced-apart locations which can be adjusted while the instrument is in the wellbore to compensate the measurement for use in inclined wellbores or for the presence of fluids in the wellbore having having particular densities which reduce the sensitivity of the tool.